Weather responsive internal roof shading systems for existing long-span glazed roof over large naturally ventilated and air-conditioned pedestrian concourses in the tropics

Abstract

This research aims are to optimize weather responsive internal roof shading design systems and to recommend some design principles and guidelines for internal roof shading systems. Such systems would then provide a better building-centric thermal environment and energy performance, while maintaining adequate levels of natural lighting within the existing long-span glazed roofs over large naturally ventilated and air-conditioned pedestrian concourse in the tropics. Two shading configurations: low and high level shadings were tested both the physical indoor environment and energy performance using dynamic thermal and lighting models on the typical clear days and overcast day in summer and winter respectively. The thermal performance of these test cases was assessed using internal surface temperatures, air temperatures, mean radiant temperatures and operative temperatures. The energy performance of the tested cases was examined using solar heat gain and cooling loads as well as the visual performance using illuminance and daylight factors. These remedial solutions were also assessed the financial benefits using standard economic analysis methods to provide recommendation on the cost and payback periods. The predicted results of the large glazed naturally ventilated pedestrian concourse reveal that the internal roof shading device was very effective in reducing inner surface temperatures and consequently reducing radiant heat gain into the space. The low level shadings are more effective than the high level shadings in term of providing better energy, internal thermal and lighting performance. This configuration would reduce two third of the solar heat gain in the large glazed pedestrian concourse space. The predicted results of the large glazed air-conditioned pedestrian concourse reveal that only the low level shading can improve physical environment in terms of thermal, energy and lighting conditions. This configuration would reduce the ground floor heat gain and also the inner surface temperatures significantly. The buffer zone is a key reason that the low level shadings perform better than the high level shading. For the naturally ventilated case, creating a ventilated naturally thermal buffer space is critical to the design of an effective internal roof shading system. The large void space between the glazed roof and the low level shadings allows the free movement of the hot air to dissipate to the outdoors at a high level before it can enter the spaces below. For the air-conditioned case, a larger volume of air over the low level shadings allow for more accumulation of heat as compared to a smaller volume of air over the high shadings. In addition, high solar reflective property of the fabric decreases the solar heat by reflect a portion of the solar heat back out through the transparent roof, while some solar energy is also trapped within the air gap. According to the thermal environmental conditions required for comfort by the operative temperature recommended by ASHRAE (2004), the both shading options of the large glazed naturally ventilated case could only ease to some degree thermal discomfort. While the low level shading of the large glazed air-conditioned case also goes a long way to alleviating summer thermal discomfort. However the shadings could reduce the internal surface temperature significantly which are the main causes of the radiation heat gain in the large glazed naturally ventilated and air-conditioned pedestrian concourses. The visual performance results in both case studies reveal that the internal roof shading significantly reduced and maintained daylighting levels at an appropriate quality of light according to the CIBSE‘s recommendation only on hot clear days. Therefore retractable shading devices are recommended to provide sun screening only when required such as on summer clear days when solar gain is likely to result in overheating. Apart from the possible financial benefits with a present interest rate at 4.85% in China and 1.35% in Thailand over a life time of 30 years, the investment of the shading system could be financially beneficial due to the NPV>0 and the IRR was greater than interest rate in both forms of long-span glazed roofs over large pedestrian concourses; natural ventilation and air-condition.